If it's so easy why don't you do it? Let me guess, you've never designed, built, or shipped any physical product.
You're like a first-year art student 'designing' the next iPhone in a 3D rendering program (Half as thick, with twice the battery life! Indestructible edge-to-edge screen!)
Update:
And keep in mind Tesla is not product of yesterday. Vibe for electronic car was around I think much longer than Tesla.But big names (Audi,BMW,etc) just ignored it for so long (maybe because of no serious competitor or corruptness -why do they need to work on something revolutionary when they are market leaders) until some guy stepped in and challeged them seriously.
Anyways that's what I read into it.
As consumers we assume hugely successful historical automotive powerhouses like BMW would have something ready to go NOW because they've been working on it already for 5 years (or ten years+ considering Prius).
Maybe not always fair, but it's generally what people are going to assume.
Maybe if the OEMs had spent the last 10 years designing and building electric cars instead of putting out diesel agents that broke the law in dozens of countries they would be in a better position.
Sure, I couldn't do it. But the difficulty has an upper bound based on a simple existence proof.